In the field of integrated circuit (IC) technology, the area of interconnect metallization has received a great deal of attention from researchers and developers over the years due to its importance within the overall IC physical structure. Although the metallization layers of an IC are used primarily to connect various circuit elements within the IC silicon, the metallization also has a direct impact on attainable device geometry, defect density, and leakage current, making IC metallization a critical area of study.
As an example, the metallization of an IC influences the density of mobile impurities within the silicon and dielectric layers that surround the interconnections.
For example, researchers have identified several metallic substances that are useful as xe2x80x9cgetteringxe2x80x9d materials, which are capable of trapping various mobile impurities in the silicon and dielectric layers of an IC. Generally speaking, gettering is a desirable process within an integrated circuit, mitigating the effects of mobile impurities that are commonly introduced during the IC fabrication process. The effects of such impurities include decreased device performance, reliability, and processing yield, among others. In general, gettering reduces these effects by restricting the movement of the mobile impurities, thus improving overall IC performance.
However, some cases exist in which too much gettering is possible; in other words, the presence of some particular types of mobile impurities, in moderate quantities, are actually beneficial to the performance of an integrated circuit. For example, it has been shown in the art that titanium is beneficial as a component of IC metallization for several reasons. As a gettering substance, titanium traps water, hydrogen, and oxygen, thus allowing these substances to be absorbed readily from within the silicon and dielectric layers of an IC. The gettering properties of titanium are discussed, for example, in Marwick, A. D., et al., xe2x80x9cHydrogen redistribution and gettering in AlCu/Ti thin filmsxe2x80x9d in Journal of Applied Physics, Vol. 69, No. 11,1 June 1991, pp. 7921-23, and Yoshimaru, M., et al., xe2x80x9cDeoxidation of Water Desorbed from APCVD TEOS-O3 SiO2 by Titanium Cap Layerxe2x80x9d in Proceedings of the 1995 IEEE International Reliability Physics Symposium, pp. 359-64. Additionally, titanium exhibits low contact resistance and helps improve electromigration properties in aluminum. In some circumstances, unfortunately, the gettering effect produced by titanium causes too many water molecules, and the hydrogen and oxygen that combine to form the water, to be absorbed within the titanium. For instance, water and its constituent elements are useful under certain conditions for passivating structural defects within silicon by bonding with the defect sites, thus causing the IC to function more efficiently. In that case, substantial gettering of hydrogen and oxygen may actually be a detriment to the performance characteristics of the IC, resulting in increased leakage current and other impediments to optimal device performance.
Therefore, in many cases it would be advantageous to construct a metallization structure that utilizes the positive qualities of titanium, such as mitigation of electromigration effects and low contact resistance, while at the same time restricting the gettering effect of the titanium so that water, hydrogen, and oxygen will be available to passivate structural defects in the silicon layer of the device.
Specific embodiments according to the present invention, to be described herein, provide a useful way for titanium to be utilized in a metallization structure of an integrated circuit. The proposed structure takes advantage of the desirable electromigration and contact resistance properties of the titanium, while at the same time limiting its gettering capabilities. Consequently, a sufficient amount of mobile impurities, such as water (and the hydrogen and oxygen it comprises), are then available to passivate structural defects in the silicon layer of the IC.
A method embodiment of the invention begins with the deposition of a layer of titanium onto a preexisting layer of an integrated circuit during fabrication of the IC. A layer of aluminum is then deposited directly onto the titanium layer. During heating of the IC that normally occurs in subsequent fabrication process steps, at least a portion of the titanium alloys with the aluminum layer. The advantage of alloying the aluminum and titanium is that the gettering capacity of the titanium is restricted, thus allowing mobile impurities such as water, hydrogen, and oxygen to be available to passivate structural defects with the silicon of the IC. At the same time, titanium still helps set the structural texture of the aluminum layer, providing low contact resistance and improved electromigration properties.
Another embodiment of the invention describes a metallization structure as it resides on a preexisting layer of the IC after the fabrication of the IC. A layer of titanium resides on the preexisting layer. A layer of aluminum then resides on top of the titanium, with the titanium layer being at least partially alloyed with the aluminum layer. Consequently, the portion of titanium that is alloyed with the aluminum is no longer available as a gettering species, while still providing the desirable electromigration and contact resistance properties normally associated with titanium.